INTERACTIONS BETWEEN MICROORGANISMS AND PLANTS

By:yusur alani2.5.2016

Outline Introduction Symbiotic Associations with Cyanobacterial Interactions in the Rhizosphere Mycorrhizae Nitrogen-Fixing Bacteria and Higher Plants Bacteria Supporting Plant Growth Leaf Surfaces and Microorganisms Detrimental Activities of Microorganisms

on Plants Fungi Promoting Increased Heat Tolerance

in Plants Biocontrol of Pests and Pathogens Summary

Introduction Cyanobacteria have symbiotic relationships with several different cellular systems. Due to cyanobacteria provide carbon materials resulting from photosynthesis to the other partner.

The rhizosphere is an environment where microbial activities contribute to plant growth. Bacteria and fungi grow on organic compounds released from the plant root and produce plant-line hormones

Atmospheric nitrogen may be reduced to ammonia by cyanobacteria and other freeliving prokaryotes

Gall production on plants is attributed to infection by Agrobacterium tumefaciens that contains the Ti plasmid

Microorganisms can be used as biocontrol agents to control agricultural pests and pathogens.

• Earth is richly populated with plants , microorganisms are also beneficial to plants,increased plant growth.

plants benefit from two types ofmicrobe–plant associations:(1) highly specialized interaction, where there is considerablespecificity found in mutalistic activities.(2) commensalism resulting from nutrient secretion from plants when bacteria and fungi grow in close proximity to the roots providing no apparent benefit to the plant. In marine environments, the number of bacteria

may reach a million/mL along the surface of red algal filaments. Because terrestrial plants do not synthesize vitamin B12.

A disease results when the natural defense systems of the plant are ineffective.

Model indicating various plant–microbe interactions

SYMBIOTIC ASSOCIATIONS WITH CYANOBACTERIA: Cyanbobacteria are broadly distributed in nature and

form symbiotic relationships withmany different organisms. cyanobacteria enhance their survival by establishing

an association with a biological partner.

Azolla: is an aquatic fern that contains bilobed leaves attached to a stem and is foundfloating in freshwater.

The cyanobacteria are found in a cavity between the ventral and dorsal epidermal layers of the leaf.

.Azolla has long been used to enrich the nitrogen level of rice fields and is often used asa fertilizer known as “green manure.

INTERACTIONS IN THE RHIZOSPHERE: The soil layer that is within a few

millimeters from the root system is the rhizosphere,which consists of soil,and all the biological agents present in the soil.

This term was introduced by Lorenz Hiltner1904 , and this designation includes bacteria, fungi, algae, protozoans, and soil animals .

Several different activities are found along the region of a growing plant root. Elongation occurs at the root tip where

mucilage is secreted and entraps materials released from the root.

The amount of carbon loss from roots is often expressed as a percentage of net carbon fixed and can be considerable

For example the carbon loss by tomato, pea, barley, and wheat has been estimated to be 35–40% of total plant carbon.

In comparing the rhizosphere to nonrhizosphere environment, the pH of the rhizosphere generally is acidic as a result of proton secretion and can be as much as 2 pH units lower than in the soil.

Not all plant systems are identical because the gene content of a cultivar selects for specific bacteria and fungi in the rhizosphere.

Mycorrhizae: Rhizospheric interactions of terrestrial fungi

are influenced by the number of microorganisms, type of microorganisms, and specific plant root exudates, some fungi in the rhizoplane may establish a mutualistic relationship with the root.

When plants are introduced into low-nutrient soil, there is an obvious benefit to plants inoculated with mycorrhizal fungi, as can be observed from growth A significant benefit to the plant with either

endomycorrhizae or ectomycorrhizae fungi is the enhanced uptake of nutrients from the soil.

The zone of nutrient absorption by root hairs is only a few millimeters, while the ectomycorrhizae may extend 20 cm or more from the root while the endomycorrhizae may scavenge for nutrients for about 8 cm from the root.

The terms mycorrhizae and mycorrhizas are plural expressions, while mycorrhiza is singular and refers to a specific root–fungus association

Ectomycorrhizae are commonly found on tree roots of

gymnosperms or woody angiosperms. Approximately6000 fungal species form

ectomycorrhizae, and ∼75% of these will produce aboveground reproductive structures commonly called “mushrooms.”

Most ectomycorrhizaefungi are basidiomycetes; however, a few are ascomycetes, and these formsubterranean reproductive structures known as “truffles.”

• Conifers and deciduous broadleaf trees (especially oak, beech, eucalyptus, and hickory)are hosts for the mycorrhizal fungi, and plants placed in nutrient-deficient soils will notgrow unless appropriate mycorrhizal fungi are added.

. roots restricted to the penetration between the epidermal cells forming an intercellularassociation with the root cortex and rarely into the root cells .Infectionby fungi into the surface layers of the root produces a compact mass that is referred toas the Hartig net.

Fungal hyphae grow extensively on the young root surface, producinga web-like mantle or sheath that may be 40 μm thick and account for 40% of the drymass of the root. The surface of a plant root covered with ectomycorrhizae.

An additional benefit of a plant having mycorrhizal fungus is the trapping of nutrients. This is observed during periods of rain when nutrients are washed from the leaf litter into the soil but are immobilized in the mycorrhizal mantle before they can be carried out of the root zone.

Generally, ectomycorrhizal fungi are incapable of digesting complex carbohydrates such as lignin or cellulose and require

sugars, organic acids, amino acids, and vitamins (especially thiamine) for growth

Endomycorrhizae : are found on most herbaceous plants,

including lower vascular plants such as ferns and bryophytes.

are the most common type of mycorrhizae because they have been demonstrated in almost every plant family.

The spores of endomycorrhizal fungi have a thick wall, and under appropriate conditions germination will produce a germ tube with an emerging fungal cell.

The germ tube must associate with a root because endomycorrhizal fungi have limited saprophytic metabolism. The fungi penetrate the root with growth between epidermal cells or through root hairs and invade cortex cells.

Other Mycorrhizal Associations there are several other morphological

types of mycorrhizae.Ectendomycorrhizae are characterized as having a relatively small fungal sheath, an extensive Hartig net, and fungal penetration into root cells.

This intracellular activity is especially obvious in older roots.

In one environment ectomycorrhizae may be produced, but in another environment the same two partners may produce ectendomycorrhizae.

In the plants of the Ericales, the symbiotic fungal association is called ericoid mycorrhizae. The mycorrhizal fungus is found in the root hairs that consist of only a few layers of cells.

The ericoid mycorrhizae are commonlyascomycetes or basidiomycetes, and their presence stimulates plant growth and promotes fruiting by the plant.

NITROGEN-FIXING BACTERIA AND HIGHER PLANTS Examples of nitrogen-fixing bacteria that grow

symbiotically with plants are given in this Table:

The enzyme system for nitrogen fixation is found only in the prokaryotes,

and in the case of symbiotic nitrogen fixation in plants, there is considerable specificity between

the legume symbiont and bacteria for the stable association.

The plant provides the carbon and energy source for the bacteria to grow, while the bacteria fix nitrogen with the production of amino acids for plant growth.

Root Associations Legume Nodules. The association of nodules with roots of

leguminous plants is one of the oldest reports in microbial ecology. In the late 1600s, Marcelo Malpighi made extensive drawings of legumes showing roots with well-developed nodules.

MartinusBeijerinck isolated bacteria from the root nodules of legume plants and established the nitrogen-fixing capability of these bacteria when associated with plants. The bacteria that grow symbiotically with roots of legumes are frequently referred to as rhizobia.

The establishment of root nodules requires bacteria that can compete in the rhizosphere as well as establish a symbiotic relationship with roots.

An important aspect for rhizospheric and symbiotic bacteria is iron nutrition, and Michael O’Connell is one of the leading experts addressing siderophore production by rhizobia

Bacteria are specific for a legume species, and formation of the root nodule is the resultof a unique differentiation process. A model representing some of the steps involved inthe formation of a nitrogen-fixing nodule are presented later

the individual steps are summarized as follows:1. The legume root secretes specific chemicals known as flavonoids ,and these signaling molecules attract rhizobia growing in the rhizosphere.

2. The flavonoids also induce transcription of nod genes in the rhizobialgenome to produce lipochitin oligosaccharides, called Nod factors.

3. The plant root recognizes the chemical structure of the Nod factors, and these lipochitin oligosaccharide molecules are taken up by legume receptor kinases. The Nod factors activate the plant hair roots, and this recognition is responsible in partfor the specificity between the host and legumes.

4. The symbiotic bacteria attach onto the root hairs and enter into the root by a processknown as root infection..

5. The bacteria produce a lipopolysaccharide capsule that enables the rhizobia to evade plant defense systems and enter the root by a structure designated as theinfection thread

6. Phytohormones are produced by bacteria to stimulate root cell division, and the plant root expands quickly to produce a nodule.7. Bacteria in the root nodule adjust to the low-nitrogen environment and the reduced oxygen level in the nodule. In some cases the bacteria differentiate to produce rounded cells or cells with a “Y” shape . These differentiated bacteriaare bacteroids, and these cellular units are important in fixing nitrogen.

A photograph of soybean roots with nodules present on the root hairs is shown in

o Nitrogen fixation in the soybean nodule is attributed to the bacteroids,differentiated rhizobial cells, present in the symbiosomes

Because the nitrogenase enzyme is oxygen-sensitive, leghemoglobin in the nodule reduces the level of free O2 to enable nitrogen fixation. The product of nitrogen fixation is the production of glutaminethis amino acid is transported by way of the root hairs to regions of protein synthesis

BACTERIA SUPPORTING PLANT GROWTH

Production of Hormones:Many rhizospheric microorganisms produce chemicals that stimulate plant growth, andthese chemicals have a molecular structure similar to that of plant hormones. many bacteria produce plant-stimulating compounds, including auxins, gibberellins, cytokinins, ethylene, and abscisic acid.

These plant-like hormones are produced by free-living, symbiotic, and pathogenic strains of rhizospheric microorganisms.

Growth-Promoting Rhizobacterian:The most numerous microorganisms in the soil are bacteria, and these organisms may influence mycorrhizal growth. Scientists recognize the importance of soil bacteria and have called them plant-growth-promoting rhizobacteria (PGPR) or simply growth-promoting bacteria. A mixture of several species of microorganisms is more

effective in stimulating plant growth than is a monoculture of bacteria or fungi.

Cactus Symbiosis: bacteria enable cactus to grow. In the harshest of

terrestrial environments. The bacteria are found as endophytes of the plant and

also are present in the cactus seeds.

o When the seeds fall on animal dung, the bacteria grow with the production of organic acids that convert rockparticles to mineral soil.

These endophytic bacteria continue to grow in the roots of the cactus with the continuous secretion of organic acids and fixation of nitrogen.

DETRIMENTAL ACTIVITIES OF MICROORGANISMS ON PLANTS:

Fungal Parasites:Several fungi and a small number of bacteria are pathogenic for plants. Fungi of all taxonomic classes are capable of causing epidemic plant diseases that produce considerable economic loss and hardship for humans

The pattern followed for disease formation in plants is generally as follows:

1. Spores or vegetative cells of fungi come in contact with the plant.2. The fungi penetrates the surface through wounds, stomata, and other openings in the plant cell.3. An infection occurs with the fungi growing in either the intracellular or intercellular region.4. Other tissues are invaded, and the pathogen is spread throughout the plant.5. Depending on the pathogen, death of the plant may be observed in a few days or a few weeks.

Bacterial Pathogens The bacteria-producing diseases in plants

generally display a number of hydrolyticenzymes for penetration of the plant surface, and growth in the plant is generally intercellular between the parenchyma cells.

As the disease progresses in the plant, bacterialdistribution may become systemic as a result of migration into the vascular tissue. In some cases, the bacteria release pectinase

that hydrolyzes the plant cell walls and the plant collapses.

..The bacterium Agrobacterium tumefaciens produces tumors in a diverse group of dicotyledonous plants at the root–stem interface that is called the “crown” of the plant.

Thus, the disease has been called “crown gall.” The microorganism is an aerobic Gram negativebacterium that is widely distributed in soils where it grows on a variety of sugars and organic acids

FUNGI PROMOTING INCREASED HEAT TOLERANCE IN PLANTS When the fungus Curvularia protuberate

infects Dichantheilum lanuginosum, a tropical grass, the plant is capable of growing in soil that has a temperature of 65◦C. However, for this heat tolerance,

the fungus must also be infected with a dsRNA virus consisting of two segments, and this virus has been designated as Curvularia thermal tolerance virus (CThTV)

BIOCONTROL OF PESTS AND PATHOGENS: the control of insects and agents responsible for

disease production in plants has been achieved through the use of chemicals.

However, an increased concern for the addition of chemicals into the environment has prompted scientists to explore theuse of biological agents to control or prevent the growth of pathogens

Plant-growth-promoting rhizobacteria (PGPR) produce a variety of antibiotic compounds that inhibit a variety of Gram-negative and Gram-positive soil bacteria,

Nematodes found in the soil are roundworms of a few millimeters in length, and some of these are parasitic for plants

As a result of nematodes attacking roots and underground parts of the plant, death of the plant may be due to direct damage by the nematode or to secondary infection by bacteria, fungi, or viruses

Certain strains of Bacillus thuringiensis produce a protein that has been used to

control insect populations and some of the insects controlled are listed in Table

Bacillus thuringiensis is an aerobic, Gram-positive bacterium found broadly distributed in soil.

SUMMARY A diverse group of microorganisms are found in

the root zone rhizosphere, and bacteria present in the rhizoplane have considerable influence on the plant

specific bacteria may enhance plant growth, due to the development of mycorrhizae, or prevent growth of phytopathogenic fungi

Fungi may be found in symbiotic association with plant roots, where the plant provides sugars and organic acids while the fungus partner enhances mineral uptake by plants

The two principal types of fungal association areendomycorrhizae found commonly on herbaceous plants, and ectomycorrhizae, which are generally associated with woody plants.

Symbiotic nitrogen-fixing plants have specificbacteria as partners for the conversion of atmospheric N2 to ammonia. Establishing the plant–bacteria activity

results from signal responses on the part of both partners in this symbiosis

A few of the fungi and bacteria are plant pathogens producing highly distinctive plant pathologies.

An important bacterial disease in plants is crown gall attributed to Agrobacteriu tumefaciens,

Unique in this infection is the presence of a Ti plasmid in the bacterium, and this specific plasmid carries genes for plant hormones to stimulate tissue proliferation.

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